It is required that a chassis frame has a high strength and stiffness. At the same time the chassis frame is required to be as light as possible. It is configured by assembling cross members and side members to have a shape of an octothorpe as disclosed in Patent Document 1. It is also configured by joining an upper member and a lower member to have a hollow cross section. The upper member and the lower member are made by a press working on a metal plate. The chassis frame is connected to a vehicle frame by bolting connection members. Each connection member is a mounting arm made of a metal pipe that is separately provided on the cross members, the side members, the upper member or the lower member.
A chassis frame disclosed in Patent Document 1 includes a flat attachment face formed by flatly crashing an end of the side member to an approximately gate shape in its cross section. The flat attachment face is on an overlapped portion where plate of the pipe (periphery of the pipe) is overlapped. The connection member further includes longitudinal ribs bending from the flat attachment face to both sides of the overlapped portion (Patent Document 1, [0006] [0008]). The flat attachment face is provided with a hole for bolting on its center. The flat attachment face is abutted to a corresponding mounted portion of a vehicle frame to bolt each other. The longitudinal ribs enhance a stiffness of an end portion of the side member and prevent it from interfering with other members by adjusting width of the flat attachment face (Patent Document 1, [0009]).